As I feared, Somerville has declared a snow emergency as of 7:30pm Saturday evening.  It no doubt will still be in effect Sunday morning before the marathon.  On-street parking for residents is only allowed on the odd-numbered side of streets.  Municipal lots will also be open to residents who can't find parking elsewhere.

What isn't clear right now is the situation for non-residents, whether they still can park on-street and in the lots during a snow emergency.  (I'd hope so, but I'm checking with the city.)

In any case, parking will be SEVERELY limited Sunday morning, as the past week has made clear.  You might want to go with public transportation if it is an option (assuming that it is running).  I will update the situation here if there is more info.  I don't believe Cambridge has declared a snow emergency at this point.